In known sensors, for example piezo-ceramic bending oscillators, electric potentials, occurring at the contact surfaces, whose amplitude depends on the amplitude of the bending motion are processed for triggering an alarm circuit permitting the vehicle operator to react accordingly or are directly used to engage the engine ignition and/or fuel metering system. The knock sensors are subject to mechanical and thermal stresses because of their intimate proximity to the internal combustion engines in which they are installed. Furthermore, they must be constructed to fit relatively confined spaces at or near the internal combustion engine. The knock sensors known heretofore in the art are not sufficiently well suited to fulfill these conditions.